Last time
• Viscosity determines the ability of a melt to
releases gasses coming out of solution
• Viscosity depends on silica content and
• TEMPERATURE
• Mafic (Basaltic) melts are extremely hot
and have low silica, do not explode much
• Felsic (Granitic) melts are only moderately
hot and have very high silica. Explode
violently
Limestone dissolved by naturally acidic rainwater
Weathering and Soils
Lecture 5
Earth’s surface processes
• Weathering – Physical breakdown and chemical
alteration of rock at Earth’s surface
• Erosion – The transport of rock fragments
Preview of Mechanical Weathering
Source: Tom Bean/DRK Photo
•
•
Mechanical weathering – breaking of rocks
into smaller pieces
Four types of mechanical weathering
1. Frost wedging – freezing and thawing of
water in cracks disintegrates rocks
Salt wedging is similar
Evidence of
Frost
Wedging in
Wheeler
Park,
Nevada
Source: Tom Bean/DRK Photo
Rockfall caused by frost wedging
ends up in a stream
Colluvium
Salt Weathering is similar, growing salt crystals in cracks
split the rock apart. At the beach and desert playas.
Unloading = Mechanical Exfoliation
Granite Batholith above subduction zone
Igneous rocks at Earth’s surface peeling like
layers off an onion due to reduction in pressure
Unloading - Exfoliation of a Pluton
Mechanical Exfoliation in
Yosemite National Park
Source: Phil Degginger/Earth Scenes
3. Thermal expansion and contraction
Alternate expansion and contraction due
to heating and cooling
- Important in deserts
Biological
Activity
Tree Roots
Growing in
Rock
Fractures
Animal
Burrows
Source: Runk/Schoenberger/Grant Heilman
Mechanical weathering
causes increase in surface area
Same volume of rock, but many pieces have much more surface area than one piece
Joint-controlled weathering
in igneous rocks
Definition: Joints vs Faults
Increases surface area
for chemical weathering
Chemical Weathering
• Breaks down rock and minerals
• Important agent in chemical
weathering is water (transports ions
and molecules involved in chemical
reactions)
• The ions form the cements in
Sedimentary Rocks
• These ions make the ocean salty
Important for metals concentration
Three types of Chemical Weathering
Dissolution of Calcite
• Dissolution
–By carbonic acid, CO2 in water
CO2 + H2O => H2CO3
H2CO3 => H+ + HCO3–Soluble ions contained in
underground water
CaCO3 +2H+ => H2CO3 + Ca++
H2CO3 => H2O + CO2
Dissolution
Weathered and
Unweathered
Limestone
Boulders
Source: Ramesh Venkatakrishnan
Oxidation
• Chemical reaction where
compound loses electrons
• Usually while bonding with oxygen
–Important in breaking down
mafic minerals (contain Fe+2 or +3)
–Rust- colored mineral (Fe2O3 )
from weathering of Basalt
[which contains Olivine (Fe, Mg) SiO4
http://www.geol.umd.edu/~jmerck/gal04/events/htmls/dauttl.html
Oxidation of Basaltic Lava Flows
Rust-colored Iron Oxide forms
Hydrolysis
• Water makes H+ and OH- ions
– Hydrolysis is the reaction of any
substance with water
– Water’s ions replace different ions
in a mineral
– Feldspars, most abundant crust minerals,
become fine clay particles.
– Clays are light weight, flat plates, easily
transported by streams
Hydrolysis – Feldspar to Clay
Feldspars become
Mechanical fracture due to chemical weathering
23
Hydrolysis
Cleopatra’s
Needle,
(Egypt)
Granite in a
Dry Climate
Source: New York Public Library, Locan
History and Genealogy Division
Hydrolysis
Cleopatra’s
Needle,
(Central
Park, NYC)
Granite in a
Wet Climate
Source: Runk/Schoenberger/Grant Heilman
Vegetation and Soil Development
Plants use Hydrolysis to get nutrient metals out of minerals
Bowens Reaction Series and Weathering
Soils
Also very important in recognizing past climates
• Soil - combination of mineral and
organic matter, water, and air
• It is that portion of the
regolith (weathered rock and
minerals) that supports the
growth of plants
Components in topsoil
that support plant growth
Factors related to bedrock composition (parent material)
Weathering-resistant sandstone (mostly quartz) yields little soil
Soil
Soil
Soil
Iron-rich
basalt
Chemical weathering
by dissolution
Limestone
Climate also very important, see below
Chemical
weathering
by oxidation
Chemical
weathering
by hydrolysis
Feldspar-rich
granite
Variations in soil development
due to topography: slope and time
Temperature
Annual precipitation
Equatorial
and tropical
rain forests
Low-latitude
deserts and
semi-deserts
Savannahs
Temperate
regions
and mixed
boreal forests
Grasslands
(steppes)
30 degrees Latitude
40ºC
1800
mm
Equator to Poles Factors
Equator
Precipitation
Temperature
30ºC
Evaporation
20ºC
Increasing depth
of weathering
600
mm
10ºC
Rainforest
Bedrock at
or very near Desert
surface
Shallow nutrients
Deep Regolith,
Deeply weathered shallow Soil
bedrock
(~40 - 50 meters deep)
US & Europe
Soil
Bedrock
Factors related to Climate
Arctic
and
tundra
regions
Soil Profile
• The soil profile
• Soil forming processes operate
from the surface downward as
water descends
• Vertical differences are called
horizons – zones or layers of soil
Soil
The soil profile
• O horizon – organic matter
• A horizon – organic and mineral matter
– High Biological Activity (animals live here)
– Together the O and A horizons make up topsoil
• E horizon – little organic matter
– Zone of leaching – soluble minerals removed
• B horizon – zone of re-precipitation
• C horizon – partly altered parent material
ONLY
ACTIVE
EDUCATORS
BECOME
CHAMPIONS
An idealized
soil profile
Remember the different horizons
ONLY
O
ACTIVE
A
EDUCATORS
E
BECOME B
CHAMPIONS
C
Organic
Animal Activity
Soluble minerals
Soluble Minerals
Exited
Back
Crushed Rock
Animal Activities in “A” horizon
Worms ingest mineral grains because they are covered with living organisms –
their food. Their burrows, not their feeding, increase chemical weathering by
exposing the minerals to water and air
Source: Runk/Schoenberger/Grant Heilman
E and B: Eluviation & Illuviation
exited
back
Illuviation and Eluviation
Leaching and Precipitation of Iron
E
B
Source: Jens/Gutzmer/Rand Afrikaans
University/Geology
Soil Types
• Soil types
• The characteristics of each soil type
primarily depend on the prevailing climatic
conditions
• Three very generic soil types
•Pedalfer
•Pedocal
•Laterite
pedalfer
Evergreen forests
pedocal
Shortgrass
laterite
tropics
Pedalfer
•Accumulation of iron oxides
and Al-rich clays in the B
horizon. Brown B horizon
•Best developed under
temperate forest landscapes
Pedocal
• White calcium carbonate
(caliche) in B horizon
• Associated with dry grasslands
and brush vegetation
Pedocal with Caliche in the B horizon
O
A
E
Organic
Activity
Leached
B
Accumulation
C
Crushed Rock
Laterite
Hot and wet tropical climates
Monsoonal Climate
• Intense chemical weathering
• Red Iron oxide - Topsoil not distinct
from B horizon –bacterium responsible for dissolving soil
iron is not present
• Deep soil but usable nutrients
shallow
Laterite in Sarawak, Borneo
The percolating rain water causes dissolution of primary rock minerals and
decrease of easily soluble elements as sodium, potassium, calcium,
magnesium and silicon. This gives rise to a residual concentration of more
insoluble elements predominantly iron and aluminum.
Source: Fletcher & Baylis/Photo Researchers, Inc.
Earth’s surface processes
• Erosion – the physical
removal of material by mobile
agents like water, wind, ice, or
gravity
Natural rates of soil erosion
depend on:
Soil characteristics
Climate
Slope
Type of vegetation
Headed for the Sea
Source: Ramesh Venkatakrishnan
Soil
• Soil erosion
• In many regions the rate of
soil erosion is significantly
greater than the rate of soil
formation
• Farmers level fields to slow
loss of topsoil
• Farmers have been
building terraces for
thousands of years
http://engineering.missouri.edu/news/2009/02/03/mizzou-engineering-helping-buildbetter-farms/